1. Field of the Invention
The present invention relates to a method of operating a phosphoric acid fuel cell having an assembly including an electrolyte impregnated with phosphoric acid and interposed between an anode electrode and a cathode electrode, in which fuel gas is supplied to the anode electrode by the aid of a fuel gas supply system, while oxygen-containing gas is supplied to the cathode electrode by the aid of an oxygen-containing gas supply system.
2. Description of the Related Art
A phosphoric acid fuel cell (PAFC) as a type of fuel cells is provided with a power-generating cell (fuel cell unit). The power-generating cell is constructed such that an electrolyte electrode assembly is interposed between separators (bipolar plates). The electrolyte electrode assembly comprises an anode electrode and a cathode electrode principally composed of carbon respectively and provided opposingly on both sides of an electrolyte matrix layer composed of a polymer membrane such as polybenzimidazole impregnated with phosphoric acid as liquid electrolyte. Usually, a predetermined number of the power-generating cells are stacked and used as a fuel cell stack.
In the fuel cell, a fuel gas such as a gas principally containing hydrogen (hydrogen-containing gas), which is supplied to the anode electrode, contains hydrogen which is ionized into ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte. The electron, which is generated during this process, is extracted to an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas, for example, a gas principally containing oxygen or air (gas containing oxygen) is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode and, thus, water is produced.
An operating temperature of the above phosphoric acid fuel cell is set to be relatively high (about 120xc2x0 C. to 190xc2x0 C.). In general, a heating means such as a heater is used to warm up the phosphoric acid fuel cell until it is steadily operated. The phosphoric acid fuel cell is operated after the temperature thereof is raised to be not less than about 100xc2x0 C. Accordingly, the self-heat generation caused by generating the power is utilized to raise the temperature up to 120xc2x0 C. to 190xc2x0 C.
While the phosphoric acid fuel cell generates the power, reaction product water exists as liquid water at a high ratio at a relatively low temperature less than 100xc2x0 C. as in starting the operation. The liquid water is hardly evaporated into steam. Therefore, concentration of the phosphoric acid is lowered by the liquid water while increasing an entire amount of the aqueous solution of phosphoric acid (hereinafter generally referred to xe2x80x9cphosphoric acidxe2x80x9d as well). Consequently, the phosphoric acid which is over a storable amount exists in the anode electrode and the cathode electrode.
If the phosphoric acid overflows from the anode electrode and the cathode electrode as described above, the phosphoric acid flows out which is retained in the electrolyte matrix layer. The flow-out phosphoric acid passes through the fuel gas flow passage and the oxygen-containing gas flow passage and is discharged out of a main body of the phosphoric acid fuel cell. Then, it is impossible for the phosphoric acid fuel cell to maintain an initial performance when it ceases operating and is restarted. As a result, there is a problem that the performance of the phosphoric acid fuel cell is worsened.
Further, if the phosphoric acid overflows in the anode electrode and the cathode electrode, an activity of the catalyst to facilitate the reaction necessary for generating the power is lowered in some cases. In addition, the fuel gas flow passage and the oxygen-containing gas flow passage are closed, thereby making it difficult to flow the reaction gas (fuel gas and/or oxygen-containing gas). As a result, there is a problem that the performance of generating the power is lowered.
If the phosphoric acid is flown out, it is assumed to use a separate apparatus for replenishing the phosphoric acid to prevent the performance from being worsened.
However, there is a problem that a size of the fuel cell system becomes inevitably large in its entirety.
It is also assumed to use a heater of a large size to quickly raise the temperature of the phosphoric acid fuel cell to be not less than 100xc2x0 C. However, there is the same problem as described above that the size of the fuel cell system becomes inevitably large in its entirety. In addition, it is not economic to use the heater of a large size.
A principal object of the present invention is to provide a method of operating a phosphoric acid fuel cell, which makes it possible to reliably prevent reaction product water from worsening the performance of the phosphoric acid fuel cell, by using a simple arrangement and control without large equipments.
In the method of operating a phosphoric acid fuel cell according to the present invention, the operating condition is set so that a phosphoric acid concentration, at which an amount of reaction product water to lower a concentration of phosphoric acid and an amount of water evaporated from phosphoric acid are equilibrated, can be not less than a reference phosphoric acid concentration to successfully maintain desired performance, when the phosphoric acid fuel cell is operated under a condition in which the reaction product water exists as liquid water.
Usually, the phosphoric acid, which is used for the phosphoric acid fuel cell, is obtained by dissolving diphosphorus pentoxide in water. Especially, the high concentration phosphoric acid, which is used for the phosphoric acid fuel cell, is highly hygroscopic, and it tends to mix with water highly easily. In this case, the vapor component of the phosphoric acid, i.e., the vapor component of the aqueous phosphoric acid solution is diphosphorus pentoxide (exactly a dimer) and water. However, the vapor pressure of diphosphorus pentoxide is greatly low up to a temperature in the vicinity of 200xc2x0 C. Therefore, although the vapor component of the phosphoric acid is substantially occupied by water, the vapor pressure of the phosphoric acid is different from that of water.
Specifically, the relationship between the saturated vapor pressure and the temperature of the phosphoric acid in the phosphoric acid concentration is shown in a diagram of FIG. 1. The curve in FIG. 1, which resides in a phosphoric acid concentration of 0%, corresponds to the saturated vapor pressure curve of water. According to FIG. 1, the saturated vapor pressure of the phosphoric acid is greatly changed depending on the phosphoric acid concentration. The water in the phosphoric acid tends to be more evaporated as the phosphoric acid concentration is getting low. By contrast, the water in the phosphoric acid is more hardly evaporated as the phosphoric acid concentration is getting high.
When the temperature of the phosphoric acid fuel cell is lower than a usual operating temperature (about 120xc2x0 C. to 190xc2x0 C.) in a low output operation and at a low temperature as in starting the operation of the phosphoric acid fuel cell, the evaporating speed of the water in the phosphoric acid is slow. As a result, an amount of the product water is larger than that of the water to be evaporated. Accordingly, the phosphoric acid concentration in the electrolyte electrode assembly is lowered while the saturated vapor pressure of the phosphoric acid is increased. Thus, the water tends to evaporate.
Accordingly, the phosphoric acid concentration is lowered depending on the amount of the product water, and the amount of the product water to lower the phosphoric acid concentration and the amount of the water evaporated from the phosphoric acid arrive at the equilibrium at a certain point of time. In this state, the amount of the phosphoric acid is increased. If the amount of the phosphoric acid is larger than that of the phosphoric acid to be stored in the electrolyte electrode assembly, there is a possibility that the phosphoric acid is discharged via the fuel gas flow passage and the oxygen-containing gas flow passage out of the phosphoric acid fuel cell. As a result, the performance of the phosphoric acid fuel cell is worsened.
Accordingly, the operating condition is set so that the phosphoric acid concentration where the amount of the reaction product water to lower the concentration of the phosphoric acid and the amount of the water evaporated from the phosphoric acid are equilibrated can be not less than the reference phosphoric acid concentration. The performance including the power generation is not worsened over the reference phosphoric acid concentration. Then, the performance of the phosphoric acid fuel cell is not worsened even if the temperature thereof is low. Thus, it is possible to efficiently generate the power.
The optimal phosphoric acid concentration depends upon an internal arrangement, a size or the like of the electrolyte electrode assembly in the phosphoric acid fuel cell. Therefore, it is necessary to experimentally preset the phosphoric acid concentration where the desired performance can be maintained.
Setting of Operating Condition
If a current density is I (A/cm2) and an electrode effective area is S (cm2), the amount of water produced per unit time, i.e., the amount of water m (mol/min) to lower the phosphoric acid concentration is constant irrespective of the fuel gas utilization factor ra (%) and the oxygen-containing gas utilization factor rc (%), and it is determined by the following expression (1).                     m        =                  I          xc3x97          S          xc3x97                      60                          96500              xc3x97              2                                                          (        1        )            
The following assumption is made. That is, the operating temperature is T xc2x0 C., the fuel gas temperature is Ta xc2x0 C., the oxygen-containing gas temperature is Tc xc2x0 C., the operating pressure on the side of the fuel gas is Pa (kPa), the operating pressure on the side of the oxygen-containing gas is Pc (kPa), the supply flow rate of the fuel gas (fuel gas flow rate) is fa (normal l/min) (provided that xe2x80x9cnormal l/minxe2x80x9d represents the flow rate as converted into the value at 0xc2x0 C., 1 atm as the normal state), the supply flow rate of the oxygen-containing gas (oxygen-containing gas flow rate) is fc (normal l/min), the flow rate of the gas discharged from the side of the fuel gas is fea (normal l/min), the flow rate of the gas discharged from the side of the oxygen-containing gas is fec (normal l/min), the ratio of the amount of water evaporated from the side of the fuel gas, of the water to lower the phosphoric acid concentration is a (%), and the ratio of the amount of water evaporated from the side of the oxygen-containing gas, of the water to lower the phosphoric acid concentration is 1-a (%).
The flow rate fea of the gas discharged from the side of the fuel gas is the sum of the flow rate of the fuel gas that is not consumed by the power generation and the amount of steam evaporated on the side of the fuel gas, of the amount of water to lower the phosphoric acid concentration (as determined by the following expression (2)).                     fea        =                              fa            xc3x97                                          100                -                ra                            100                                +                      m            xc3x97                          a              100                        xc3x97            22.4                                              (        2        )            
Accordingly, when the fuel gas temperature Ta xc2x0 C. and the oxygen-containing gas temperature Tc xc2x0 C. are equal to the operating temperature T xc2x0 C., the saturated vapor pressure Ps, a of the phosphoric acid in the discharged fuel gas, which is obtained at the point of time at which the amount of product water to lower the phosphoric acid concentration and the amount of water evaporated from the phosphoric acid are equilibrated, is determined by the following expression (3).                                                         Ps              ,                              a                =                                                                            m                      xc3x97                                              a                        100                                            xc3x97                      22.4                                        fea                                    xc3x97                  P                  ⁢                                      xe2x80x83                                    ⁢                  a                                                                                                        =                                                                    m                    xc3x97                                          a                      100                                        xc3x97                    22.4                                                                              fa                      xc3x97                                                                        100                          -                          ra                                                100                                                              +                                          m                      xc3x97                                              a                        100                                            xc3x97                      22.4                                                                      xc3x97                P                ⁢                                  xe2x80x83                                ⁢                a                                                                        (        3        )            
By contrast, the flow rate fec of the gas discharged on the side of the oxygen-containing gas (which is the air containing 21 wt % of oxygen and 79 wt % of nitrogen) is determined by the following expression (4).                     fec        =                              fc            xc3x97                          (                                                0.21                  xc3x97                                                            100                      -                      rc                                        100                                                  +                0.79                            )                                +                      m            xc3x97                                          100                -                a                            100                        xc3x97            22.4                                              (        4        )            
Further, the saturated vapor pressure Ps, c of the phosphoric acid in the discharged oxygen-containing gas is similarly determined by the following expression (5).                                                         Ps              ,                              c                =                                                                            m                      xc3x97                                                                        100                          -                          a                                                100                                            xc3x97                      22.4                                        fec                                    xc3x97                  Pc                                                                                                        =                                                                    m                    xc3x97                                                                  100                        -                        a                                            100                                        xc3x97                    22.4                                                                              fc                      xc3x97                                              (                                                                              0.21                            xc3x97                                                                                          100                                -                                rc                                                            100                                                                                +                          0.79                                                )                                                              +                                          m                      xc3x97                                                                        100                          -                          a                                                100                                            xc3x97                      22.4                                                                      xc3x97                Pc                                                                        (        5        )            
The relationship between the saturated vapor pressure of the phosphoric acid and the phosphoric acid concentration is shown in a diagram in FIG. 2. The concentration of the phosphoric acid at the temperature T xc2x0 C. is determined as a function of the saturated vapor pressure of the phosphoric acid. As clearly understood from FIG. 2, at a certain operating temperature T xc2x0 C., the lower the saturated vapor pressure of the phosphoric acid is, the higher the phosphoric acid concentration is. Therefore, the phosphoric acid concentration, at which the amount of the product water to lower the phosphoric acid concentration and the amount of the water evaporated from the phosphoric acid are equilibrated at the operating temperature T xc2x0 C., can be determined by using the saturated vapor pressure Ps, a of the phosphoric acid in the discharged fuel gas and the saturated vapor pressure Ps, c of the phosphoric acid in the oxygen-containing gas respectively.
Specifically, the saturated vapor pressure Ps, a of the phosphoric acid in the discharged fuel gas is a function of the operating pressure Pa on the side of the fuel gas and the flow rate fea of the gas discharged from the side of the fuel gas. The flow rate fea of the discharged fuel gas described above is a function of the amount of the water m to lower the phosphoric acid concentration and the utilization factor ra of the fuel gas. If m is constant, the lower the utilization factor ra of the fuel gas is, the larger the flow rate fea of the discharged fuel gas. Therefore, the saturated vapor pressure Ps, a of the phosphoric acid in the discharged fuel gas is low when the operating pressure Pa on the fuel gas side is low or when the utilization factor ra of the fuel gas is low. As a result, the phosphoric acid concentration is increased.
Similarly, the saturated vapor pressure Ps, c of the phosphoric acid in the discharged oxygen-containing gas is a function of the operating pressure Pc on the oxygen-containing gas side, the utilization factor rc of the oxygen-containing gas, and the flow rate fc of the oxygen-containing gas as converted into the value at 0xc2x0 C., 1 atm as a normal state. Therefore, the saturated vapor pressure Ps, c of the phosphoric acid in the discharged oxygen-containing gas is low when the operating pressure Pc on the oxygen-containing gas side is low or when the utilization factor rc of the oxygen-containing gas is low. As a result, the phosphoric acid concentration is increased.
Accordingly, by setting the utilization factor ra of the fuel gas and the utilization factor rc of the oxygen-containing gas and/or the operating pressure Pa on the fuel gas side and the operating pressure Pc on the oxygen-containing gas side, the phosphoric acid concentration, at which the amount of the product water to lower the phosphoric acid concentration and the amount of the water evaporated from the phosphoric acid are equilibrated, is not less than the reference phosphoric acid concentration. Over the reference phosphoric acid concentration, the performance is not worsened, i.e., the desired performance can be maintained. Thus, it is possible to reliably prevent the performance from being worsened even in the operation at a low temperature.
By contrast, the phosphoric acid concentration is determined as the function of the saturated vapor pressure of the phosphoric acid and the temperature (see FIG. 2). Therefore, when the saturated vapor pressure of the phosphoric acid is constant, it is possible to increase the phosphoric acid concentration at the equilibrium if the temperature, at which the amount of the product water to lower the phosphoric acid concentration and the amount of the water evaporated from the phosphoric acid are equilibrated, is set to be high.
That is, the phosphoric acid temperature T xc2x0 C. at the equilibrium interface, at which the amount of the product water to lower the phosphoric acid concentration and the amount of the water evaporated from the phosphoric acid are equilibrated, is made to be higher than the operating temperature T xc2x0 C. by making the fuel gas temperature Ta xc2x0 C. and the oxygen-containing gas temperature Tc xc2x0 C. to be higher than the operating temperature T xc2x0 C. Thus, the phosphoric acid concentration at the equilibrium can be made to be high. The relationship between the fuel gas temperature Ta xc2x0 C. and the oxygen-containing gas temperature Tc xc2x0 C. and the phosphoric acid temperature Txe2x80x2 xc2x0 C. at the equilibrium interface is inherent in the concerning phosphoric acid fuel cell, which is desirably determined experimentally beforehand.